Gallium nitride (GaN) III-V compound semiconductor-based light-emitting diodes (LEDs) typically exhibit excellent light emission characteristics. Theoretically, LED emission from GaN-based III-V compound semiconductors, such as InGaN, AlGaN, AlInGaN, and GaN, can cover the entire visible light spectrum from short wavelengths (i.e., UV) to longer wavelengths (i.e., red light).
Blue and green GaN-based LED's have been extensively utilized in industrial applications, and presently white light GaN-based LED's are attracting increased attention for use in display and lighting applications. Currently, white light emitting GaN-based LEDs may be fabricated in several different ways. One approach for fabricating white light emitting GaN-based LEDs is to combine a phosphor-based wavelength converter with a GaN-based LED which emits UV or blue light. According to this approach, some or all of the UV or blue light emitted by the LED is absorbed by the phosphor material(s) and re-emitted as longer wavelength light. White light is generated when the phosphor material(s) re-emit light of lower energies (longer wavelengths) and one or more different wavelength bands of the re-emitted light combine to form white light. One type of phosphor-based wavelength converter utilized in such applications is a cerium (Ce)-doped yttrium-aluminum garnet (YAG:Ce) material.
According to another approach for fabricating white light emitting GaN-based LEDs, devices are formed comprising a pair of active, i.e., light-emitting, regions, e.g., a InGaN-based primary, blue light emitting active region and an AlGaInP-based secondary, light converting region. In operation, a fraction of the blue light emitted by the InGaN-based primary, blue light emitting active region is absorbed by the AlGaInP-based secondary, light converting region and re-emitted as lower energy (longer wavelength) photons. White light is perceived as emitting from the device when the two light sources have an appropriate intensity ratio and wavelengths.
Still another approach for fabricating white light emitting LEDs involves combining two or more different LEDs, e.g., red, green, and blue LEDs, wherein each LED semiconductor chip is provided with its own current supply. The LEDs emit photons at selected different wavelengths and power ratio resulting in perceived white light. A drawback of this approach is the requirement for complex driving circuitry for operating the LEDs in concert and the large package size.
According to yet another approach for fabricating GaN-based white light emitting LEDs, two complementary-wavelength active LED junction regions are formed in series on a single substrate, e.g., consisting of an InGaN/GaN multiple quantum well (MQW) region with low indium (In) concentration and an InGaN/GaN MQW region with high indium (In) concentration. The former MQW region provides LED emission of blue light and the latter MQW region provides LED emission with green light, the combination being perceived as white light.
However, fabrication of white light emitting GaN-based LEDs according to each of the above-described approaches entails increased manufacturing complexity, cost, and specific drawbacks such as poor device reliability. Accordingly, it is considered that a new approach for fabrication of white light emitting GaN-based LEDs which offers simple processing and low manufacturing cost not requiring expensive post processing wafer bonding and packaging is desirable. In addition, there is a need for improved GaN-based white light emitting LEDs which avoid the need for short lifetime phosphor-based wavelength converters and thus exhibit superior reliability, improved power utilization efficiency, lower forward operating voltage, and little or no spectrum shifts when the emitted light is viewed at different distances and angles.